1. Field of the Invention
The present invention relates to a vehicle body frame of a motorcycle.
2. Description of the Related Art
A motorcycle has a vehicle body frame and a known main frame constituting a conventional vehicle body frame of a motorcycle is formed of a front frame and a rear frame separated from each other, and connected by means of welding or the like so as to provide an integrated main frame structure of a motorcycle.
For example, Patent Document 1 (Japanese Patent Application Laid-Open Publication No. 2004-114719) discloses a vehicle body frame of a motorcycle in which front ends of rear arm brackets are welded to rear ends of a pair of left and right tank rails.
Patent Document 2 (Japanese Patent Application Laid-Open Publication No. 6-107261) also discloses a frame structure of a motorcycle in which volume ratios of a front frame and a main frame are substantially the same. In such frame structure, a joining portion between the front frame and the main frame is formed into a shape of a matching surface of a jigsaw puzzle, and a joining length of the joining portion is designed so as to be longer than a joining length necessary and sufficient for obtaining strength as a vehicle body frame of a motorcycle.
For a main frame constituted by a front frame and a rear frame separated from each other, the front frame and the rear frame are each formed by casting or the like, and then joined together by means of welding, for example. At this time, it is preferred that connecting surfaces (joining surfaces) of the front frame and the rear frame are accurately abutted against each other and then joined.
However, as in the vehicle body frame disclosed in Patent Document 1, in a structure in which a front frame (or a rear frame) is increased in size, it is difficult to accurately position or align the respective jointing surfaces, when connected.
Moreover, depending on a shape or a position of the jointing surface, there is a fear such that each frame may be deformed by influence of thermal stress in a welding process. In particular, when the joining portion between the frames has a complicated matching surface as in the frame disclosed in Patent Document 2, such deformation noticeably occurs, thus being inconvenience and undesirable.
Furthermore, such deformation and the difficulty in positioning may result in displacement of a jointed main frame in appearance or variations between individual frames, thus being also inconvenient.